The development of the idea of prosthetic vascular grafts has been a major goal of vascular surgery since the first grafts were used over 30 years ago. Most approaches have concentrated on creating a surface that is thromboresistant, with the majority of these efforts directed toward an improved polymer surface. Perhaps the ideal blood-surface interface is the naturally occurring human endothelium. If present on a prosthetic graft, it would offer many of the advantages of a native vessel. Unfortunately, endothelialization occurs only to a limited degree in prosthetic grafts when placed into humans, in contrast to animals where graft endothelialization does occur. Seeding endothelial cells onto preclotted prosthetic grafts prior to implantation has improved the endothelial cell coverage of grafts in animals, but this technique has had limited use in humans. See "Human Adult Endothelial Cell Growth in Culture", Bruce Jarrell, et al., Journal of Vascular Surgery 1984, I(6), 757-764; Herring et al., "A Single and Staged Technique for Seeding Vascular Grafts with Autogenous Endothelium", Surgery 1978, 84, 498-504; Graham et al., "Cultured Autogenous Endothelial Cell Seeding of Vascular Prosthetic Grafts", Surg Forum 1979, 30, 204-6; Graham et al., "Expanded Polytetrafluoroethylene Vascular Prostheses Seeded with Enzymatically Derived and Cultured Canine Endothelial Cells", Surgery 1982, 91, 550-9 and Dilley et al., "Endothelial Seeding of Vascular Prostheses", Biology of Endothelial Cells, pp 401-11, Jaffe ed., The Hague: Martinus Nijhoff, 1984.
Over the past three decades, artificial grafts have been used to provide immediate restoration of blood flow to areas of ischemia as a result of atherosclerotic vascular disease. In addition, they have been used to provide vascular access for hemodialysis in patients with chronic renal failure, and in the repair of arterial aneurysms. Although initially successful at restoring perfusion to ischemic tissues, the long-term prognosis for these grafts is not encouraging. Over an extended period, grafts less than 4 mm in diameter lose their patency as they become occluded via fibrin deposition and cellular adhesion. Dilley supra. This process appears to be secondary, and to be due in part to the thrombogenic nature of the nude (i.e., nonendothelialized) surface of the implanted prostheses. See Berger et al., "Healing of Arterial Prostheses in Man: It's Incompleteness", Ann. Surg. 1972, 175, 118-27. Thus, much current research is being aimed at either: (1) developing grafts with an artificial, non-thrombogenic surface, or (2) lining vascular prostheses with human endothelial cells, in the hope of producing a non-thrombogenic endothelial cell surface such as exists in native human vessels.
Endothelial cells from animal sources have been studied in culture since the 1920's. In 1973, Jaffe et al. successfully cultured endothelial cells from human umbilical veins and these cells have been characterized functionally. See Jaffe et al., "Synthesis of Antihemophilia Factor Antigen by Cultured Human Endothelial Cells", J. Clin. Invest. 1973, 55, 2757-64; Lewis, "Endothelium in Tissue Culture", Am. J. Anat. 1922, 30, 39-59; Jaffe et al., "Culture of Human Endothelial Cells Derived From Umbilical Veins", J. Clin. Invest. 1973, 52, 2745-56. These cell cultures demonstrate a growth potential, but the total number of cells produced from a single umbilical vein is usually quite limited, in the range of a 10-100-fold increase in harvested endothelial cells.
While several techniques have been proposed to increase the number of cells produced in the use of human umbilical vein endothelial cells, the ability to culture endothelial cells in large numbers remains less than ideal. Some investigators have had some success in culturing human adult endothelial cells from pulmonary arteries and veins, but only for short periods of time. It has also been shown that human iliac artery endothelial cells may be cultured for a short number of passages. In a study by Glassberg et al., for example, it is reported that 50 to 500 viable cells can be obtained per 5-inch vessel segment, a very low yield. "Cultured Endothelial Cells Derived From Human Iliac Arteries", In Vitro 1982, 18, 859-66. Fry et al. have reported successfully culturing human adult endothelial cells from abdominal arteries removed at the time of cadaver donor nephrectomy, but these cells also demonstrated early senescence.
It is apparent from existing techniques that it is difficult to produce enough cells to preendothelialize a graft with a reasonable amount of vessel from the donor patient. Rather than completely endothelializing a graft prior to implantation, the concept of subconfluent "seeding" of a preclotted graft developed. Seeding vascular grafts with autogenous endothelial cells has recently been shown to increase the rate of endothelial coverage of the grafts of experimental animals. See Herring et al. and Graham et al. supra. Once covered by endothelium, grafts in dogs have been shown to be less thrombogenic as measured by platelet reactivity, to be more resistant to inoculation from blood-born bacterial challenge, and to have prolonged patency of small-caliber vascular grafts. See Sharefkin et al., "Early Normalization of Platelet Survival by Endothelial Seeding of Dacron Arterial Prostheses in Dogs", Surgery 1982, 92, 385-93; Stanley et al., "Enhanced Patency of Small Diameter Externally Supported Dacron Iliofemoral Grafts Seeded with Endothelial Cells", Surgery 1982, 92, 994-1005; and Watkins et al., "Adult Human Saphenous Vein Endothelial Cells: Assessment of Their Reproductive Capacity for Use in Endothelial Seeding of Vascular Prostheses", J. Surg. Res. 1984, 36, 588-96.
A point of major concern when translating to human graft seeding has been the ability to produce enough endothelial cells with the use of human vascular tissue to allow seeding at a density high enough to attain endothelial coverage of the graft. Watkins et al., using human saphenous vein remnants following coronary artery bypass surgery were able to produce small quantities of endothelial cells in culture, and reported a low-fold increase in confluent cell area obtained in culture after 4 to 6 weeks. See Watkins et al. supra.
Even if it were possible to substantially expand the number of endothelial cells available through vigorous culturing techniques, concerns would still remain concerning the "health" of these endothelial cells after as many as 40 or 50 population doublings. Furthermore, the incubation of such cells in cultures which are foreign to their natural environment raises further concerns about genetic alterations and/or patient contamination with viruses, toxins or other damaging materials.
Many endothelialization procedures are suggested in the literature. Investigations in this area have been complicated by the diverse nature of the endothelium itself, and by the species to species differences which have been found relating to the behavior and characteristics of the endothelium. Fishman, "Endothelium: A Distributed Organ of Diverse Capabilities", Annals of New York Academy of Sciences 1982, 1-8; Sauvage et al., "Interspecies Healing of Porous Arterial Prostheses", Arch Surg. 1974, 109, 698-705; and Berger, "Healing of Arterial Prostheses in Man: Its Incompleteness", supra. Nonetheless, the literature is replete with reports of experiments involving the seeding of endothelial cells on various grafts, in various species, with a mixture of results. F. Hess et al., "The Endothelialization Process of a Fibrous Polyurethane Microvascular Prostheses After Implantation in the Abdominal Aorta of the Rat", Journal of Cardiovascular Surgery 1983, 24(5), 516-524); W. K. Nicholas et al., "Increased Adherence of Vascular Endothelial Cells to Biomer Precoated with Extracellular Matrix", Trans. Am. Soc. Artif. Intern Organs 1981, 28, 208-212; C. L. Ives et al., "The Importance of Cell Origin and Substrate in the kinetics of Endothelial cell Alignment in Response to Steady Flow", Trans. Am. Soc. Artif. Intern Organs 1983, 29, 269-274; L. M. Graham et al., "Expanded Polytetrafluoroethylene Vascular Prostheses Seeded with Enzymatically Derived and Cultured Canine Endothelial Cells", Surgery 1982, 91 (5), 550-559; S. G. Eskin et al., "Behavior of Endothelial Cells cultured on Silastic and Dacron Velour Under Flow conditions" In Vitro: Implications for Prelining Vascular Grafts wit Cells , Artificial Organs 1983, 7 (1), 31-37; T. A. Belden et al., "Endothelial Cell Seeding of Small-Diameter Vascular Grafts", Trans. Am. Soc. Artif. Intern. Organs 1982, 28, 173-177; W. E. Burkel et al., "Fate of Knitted Dacron Velour Vascular Grafts Seeded with Enzymatically Derived Autologous Canine Endothelium", Trans. Am. Soc. Artif. Intern. Organs 1982, 28, 178-182; M. T. Watkins et al., "Adult Human Saphenous Vein Endothelial Cells: Assessment of Their Reproductive Capacity for Use in Endothelial Seeding of Vascular Prostheses", Journal of Surgical Research 1984, 36, 588-596; M. B. Herring et al., "Seeding Arterial Prostheses with Vascular Endothelium", Ann. Surg. 1979, 190(1), 84-90; A. Wesolow, "The Healing of Arterial Prostheses--The State of the Art", Thorac. Cardiovasc. Surgeon 1982, 30, 196-208; T. Ishihara et al., "Occurrence and Significance of Endothelial Cells in Implanted Porcine Bioprosthetic Valves", American Journal of Cardiology 1981, 48, 443-454; W. E. Burkel et al., "Fate of Knitted Dacron Velour Vascular Grafts Seeded with Enzymatically Derived Autologous Canine Endothelium", Trans. Am. Soc. Artif Intern Organ 1982, 28, 178-182.
It has been previously recognized that human microvascular endothelial cells, that is, the cells which are derived from capillaries, arterioles, and venules, will function suitably in place of large vessel cells even though there are morphological and functional differences between large vessel endothelial cells and microvascular endothelial cells in their native tissues.
U.S. Ser. No. 725,950, filed Jun. 27, 1991, described the treatment to confluence of a vascular graft or other implant using microvascular endothelial cells which are separated from fat which is obtained at the beginning of an uninterrupted surgical procedure. Fat tissue is removed from the patient after sterile conditions have been established. Microvascular endothelial cells in that fat are then quickly separated from their related tissue by enzymatic digestion and centrifugation, and the cells are deposited on a surface by gravity or by filtration, which surface is then implanted in the patient during the latter stages of the same operation.
Notwithstanding the work reported in this field, a need still exists for improved grafts, simple, reliable procedures which can successfully endothelialize the surfaces of human implants such as surfaces of vascular grafts, and for other methods of vascularization.